This invention is related to the determination of the position of a cement wiper plug in a pipe string run in the wellbore during the cementing of this pipe string in a wellbore, and is more particularly directed to the indication of the arrival of a cement wiper plug at a predetermined position in this pipe string during these cementing operations.
In the drilling of wellbores for the production of hydrocarbons from subsurface formations, cement is pumped into the annulus between the pipe string run in the wellbore and the wall of the wellbore to both secure the pipe string in the wellbore, and to prevent communication between different subsurface formations from occurring in the annulus between the pipe string and the wall of the wellbore. In placing this cement in the annulus, it is common practice to pump a volume of cement slurry sufficient to fill this annulus down the pipe string and then to displace this volume of cement slurry by pumping a displacing fluid, such as drilling mud behind the cement slurry. Generally, a cement wiper plug is used to separate the cement slurry from the displacing fluid, and to wipe the interior surface of the pipe string clean of cement. This cement wiper plug, by seating in a landing assembly, either a float collar or landing collar, located near the bottom of the pipe string also serves to seal the annulus from the pipe string and prevent any pumping of displacing fluid from the pipe string into the annulus after the cement slurry has been placed in the annulus. In this way, the possibility of there being either displacing fluid instead of cement slurry or a cement slurry contaminated with displacing fluid in the lower portion of the annulus between the pipe string and the wall of the wellbore is reduced. Further, the seating of the cement wiper plug in the landing assembly causes an increase in the pipe string pressure which provides an indication that all of the cement has been displaced from the pipe string into the annulus between the pipe string and the wall of the wellbore, thus indicating that the cementing operation has been completed. Any backflow of cement from the annulus between the pipe string and the wall of the wellbore to the pipe string can be prevented by the utilization of either a float collar and/or a pipe string cement shoe, containing one way flow check valves. This pipe string run in the wellbore can be either a liner run from a hanger within a larger pipe string to the lower bottom hole end of the wellbore or casing string run from the surface end of the wellbore to the lower bottom hole end on the wellbore.
In a liner cementing operation, the volume of cement slurry to be used to fill the annulus between the wall of the wellbore and the liner with cement, is introduced into the liner through a smaller diameter drill string. A drill string wiper plug, released from the surface, is used to separate the cement slurry slug from the displacing fluid, and to wipe the interior of the drill string clean of cement. This drills string wiper plug is displaced through the drill string with a displacing fluid pumped into the drills string from the surface. In the case of a liner cementing operation, a larger second plug is held in place at the lower end of the drill string, but inside the liner by a latching mechanism. The cement is displaced through a center bore in this second plug into the liner and from there into the annulus between the liner and the wellbore. The drill string wiper plug seats in this second plug in a sealing relationship to form a single cement wiper plug, and the latching mechanism is released, causing the cement wiper plug to be displaced behind the cement slurry slug in the liner, thus displacing this cement through the liner, separating the displacing fluid from the cement slurry, and wiping the interior of the liner clean of cement. This cement wiper plug seats in a landing assembly located near the bottom of the liner so as to prevent any pumping of displacing fluid from the liner to the annulus between the liner and the wall of the wellbore. In this way, the displacing fluid in the liner is prevented from displacing or commingling with the cement in the annulus. Further, an indication of the end of the liner cementing operation is provided by the increase in the drill string pressure when the cement wiper plug seats in the landing assembly.
In these liner cementing operations, it is advantageous to know the location of the cement wiper plug in the liner relative to the volume of displacing fluid that has been pumped downhole, as a form of a check point in the liner cementing operation. The provision of check points is important since the displacement of the cement wiper plug to the landing assembly is critical. In the case where displacing fluid has gotten past the cement wiper plug to the cement side of the cement wiper plug, a displacement of the cement wiper plug to the landing assembly would result in the displacing fluid either replacing or commingling with and contaminating the cement slurry around the bottom downhole end of the liner. Thus, in such a case, it is important to provide a check of cement wiper plug location for a given volume of displacing fluid pumped to determine if any displacing fluid has gotten past the cement wiper plug to the cement side of the cement wiper plug and also to provide an indication of the additional volume of displacing fluid that must be pumped to fully seat the cement wiper plug in the landing assembly.
In current practice, an increase in drill string pressure is sometimes observed, when the drill string wiper plug engages the liner wiper plug prior to the release of the latching mechanism holding the liner wiper plug in place. The increase in drill string pressure is used to provide an indication of the location of the cement wiper plug and from this, both a check of wiper plug location for a given volume of displacing fluid pumped and a cement wiper plug location determination from which the volume of displacing fluid required to displace the cement wiper plug to the landing assembly are provided. However, it has been observed that in cementing deep, long liners, frequently the cement slurry is on a free-fall down the drills string and liner, creating a vacuum in the drill string near the surface behind the free-falling column of fluids, which causes the liner wiper plug latching mechanism to release when the drill string wiper plug contacts it, without an observed increase in drills string pressure. This free-fall phenomena has been described by R. M. Beirute in SPE Paper 13045, "The Phenomenon of Free Fall During Primary Cementing." Because of this lack of an observed increase in drill string pressure, there is no indication of the position of the cement wiper plug from which both a check of wiper plug location for a given volume of displacing fluid pumped and a determination of the volume of displacing fluid sufficient to seat the cement wiper plug in the landing assembly can be obtained.
In the case of casing string cementing operations, the cement slurry is introduced into the casing string and a cement wiper plug is used to separate the cement slurry from the displacing fluid used to displace the cement slurry slug down the casing string and up the annulus between the casing string and the wall of the wellbore, and to wipe the interior of the casing string clean of cement. It is advantageous to know the volume of fluid required to properly seat the cement wiper plug in a landing assembly located near the bottom of the casing string, so as to prevent any pumping of displacing fluid from the casing string to the annulus. In this way the above-mentioned problems of the displacing fluid replacing or commingling with the cement in the annulus between the wall of the wellbore and the casing string are avoided. Further, this provides an indication of the end of the casing string cementing operation through the increase in the casing string pressure.